This invention relates to an apparatus for sensing misfiring in an internal combustion engine based on an ionic current generated between electrodes of a spark plug corresponding to each of a plurality of cylinders and, more particularly, it relates to sensing a failure in an ionic current sensing unit. More specifically, the invention concerns with a misfiring sensing apparatus for an internal combustion engine, in which individual constituent means are simplified in construction for cost reduction.
In internal combustion engines generally used for automotive vehicles or the like, a plurality of (e.g., four) cylinders, which are driven to operate in synchronism with the rotation of a crankshaft, are repeatedly controlled by an engine control unit (ECU) including a microcomputer to perform four cycles consisting of intake, compression, combustion and exhaust strokes. At this time, unless an air/fuel mixture in a cylinder compressed by a piston therein is combusted in an optimum fashion and without fail in the combustion cycle, abnormal loads are applied to the other cylinders. In such a case, it is liable to cause damage to the engine and give rise to various troubles or problems due to the leakage of uncombusted gases.
When misfiring in a cylinder is sensed, the fuel supply to this cylinder is stopped or other like measures are taken for preventing the exhaust gas processing catalyst in a catalytic converter from being damaged or degraded by uncombusted gases. Thus, in order to avoid adverse effects on the internal combustion engine and the catalyst, it is necessary to check whether combustion has been done for each cylinder without fail. Heretofore, there has been proposed an apparatus which senses ionic current generated in a gap between electrodes of a spark plug during combustion of an air/fuel mixture, and determines the occurrence of misfiring if the ionic current thus generated is below a predetermined level.
FIG. 8 is a circuit diagram showing a usual misfiring sensing apparatus for an internal combustion engine. The circuit shown in the Figure is only for one cylinder. Actually, identical circuits are provided independently for respective cylinders.
The apparatus illustrated includes a power supply 1 connected to an unillustrated battery, a spark plug 2 having a primary winding 2a with one end thereof connected to the power supply 1 and a secondary winding 2b, a power transistor 3 connected between the primary winding 2a and ground, and a reverse current checking diode 4 with a cathode thereof connected to the secondary winding 2b.
The spark plug 5 has a pair of opposed electrodes, one of which is connected to the secondary winding 2b via the diode 4, the other electrode being grounded. Though not illustrated, a spark plug is provided for each of a plurality of cylinders, with its electrodes disposed in a combustion chamber defined in each cylinder.
A power supply 6 is connected to an anode of the reverse current checking diode 4. The diode 4 is connected between the power supply 6 and a junction between a diode 7 and the spark plug 5. An output terminal 9 for sensing an ionic current is connected to a junction between the power supply 6 and a resistor 8.
The operation of the misfiring sensing apparatus as shown in FIG. 8 will now be described with reference to a waveform diagram illustrated in FIG. 9.
In the combustion or ignition cycle, when a primary current I1 in the primary winding 2a is cut off under the control of the power transistor 3 in response to a control signal C from an ECU (not shown), a secondary voltage V2, which is a high negative voltage, is induced. As a result, a discharge spark is produced between the electrodes of the spark plug 5 to thereby cause combustion of the fuel in the combustion chamber. The discharge at this time is usually continued for 1 to 1.5 msec.
When normal combustion is effected in the combustion cycle, a large quantity of cations are generated. These cations flow as an ionic current I from one of the electrode of the spark plug 5 through the diode 7 to the power supply 6 and thence through the resistor 8 to ground. Thus, by sensing a voltage drop across the resistor 8, the level of the ionic current I thus generated can be sensed to determine whether normal combustion has been effected.
The data about the sensed level of the ionic current I is output from the output terminal 9 to the unillustrated ECU, which determines, based thereon, whether normal combustion has been effected in the cylinder that has been fired by the spark plug 5. If misfiring is determined, the sparking or ignition timing can be properly controlled in a feedback manner, or other appropriate measures such as stopping the fuel supply to the misfiring cylinder, stopping the operation of the misfiring cylinder or the like can be taken.
In the prior art misfiring sensing apparatus described above, however, upon occurrence of breaks or short-circuiting in wiring connecting between each cylinder and an ionic current sensing means inclusive of the elements 6 through 9 as well as in a transmission line connecting between the output terminal 9 and the ECU, or upon a failure in a circuit element in the ionic current sensing means, it becomes difficult to sense the ionic current I. In this case, the ECU erroneously determines misfiring in the engine.
Further, in a special operating range of the engine such as an engine start-up operation, idling or quick acceleration, a cylinder is sometimes supplied with no fuel even in the normal operating condition of the engine. In such a case, an ionic current I fails to be sensed, so the ECU erroneously determines misfiring in the engine. This is attributable to such causes as an unstable rotating state of the engine during an engine start-up period, a low output at the time of idling, incapability of supplying a sufficient amount of fuel to cylinders upon sudden acceleration, cut-off of the fuel supply at the time of sudden acceleration and so forth.
As described above, if the ionic current I fails to be sensed in spite of normal combustion occurring in the engine, the ECU determines misfiring, and a display means displays an indication of misfiring or abnormality, thus giving unnecessary warning or uneasiness to the driver.
In a further aspect, in the prior art misfiring sensing apparatus described above, the level of the ionic current I is sensed by a misfiring determining section in the ECU, so complicated signal processing and calculations have to be performed in the misfiring determining section. It is thus impossible to realize simplification and cost reduction of the misfiring determining section.
Furthermore, the above-described apparatus requires a plurality of transmission circuits used for sensing an ionic current I for each cylinder, and a crank angle sensor for identifying each cylinder. This results in a complicated circuit arrangement and increased noise superimposition.